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336                              CHEMICAL ENGINEERING
The chief feature of the Wetherill machine consists of two magnetic poles placed in very close contact with one another. A main conveyor belt carrying the material to be separated passes between the poles. Cross belts at right angles to the main belt also pass between the poles for carrying off the magnetic material. These are in contact with the upper pole and the magnetic material clings to the lower side until without the influence of the magnet. The upper pole is bevelled so as to concentrate the lines of force and is the effective attractive pole. In zinc-cleaning problems the Wetherill may require as much as four amperes of current, with 30,000 ampere-turns. The capacity of the Wetherill ranges from 700 to 1000 Ib. per hour and the machine will consume 7.5 hp.
In the International separator a cylindrical armature made up of thin laminated discs of a special annealed wrought iron and mounted on a steel shaft revolves horizontally between the pole pieces of a large inverted horseshoe field magnet. By induction the magnetism of the pole pieces causes magnetic poles to appear on either side of the surface of the armature. Material to be separated is fed from the hopper upon the top of the revolving armature and is carried by its movement around the armature. At the bottom position where the polarity changes the last of the material clinging to the armature will fall off as there is no attraction there whatever. The magnetic particles will fall off through an arc of 90 deg. in the order of their attract-ability and by suitably arranged hoppers below separations can be made taking advantage of attractive differences. In the Wetherill machine three sets of poles effect the same purpose. The Wetherill machine in the test size is very convenient for testing as it can be readily adjusted for giving high or low intensities.
The range of magnetic separators is about the same as shaking tables, from 12 to 80 mesh. They fail on dust or very fine material. The Western ore mills they are most commonly used for effecting the separating the mixed pyrites-blende product which comes from shaking tables. This owing to the slight specific gravity differences always comes off the shaking tables badly intermixed. If the blende be ferruginous it may be possible to separate it from the pyrities. Usually however it is necessary to roast it to magnetic pyrites after which a low intensity machine of the Dings or other type may be used. It is very difficult to roast pyrites and get the resulting product all into the magnetic iron form. Roasting and treatment of table products requires dryers, roaster, coolers, magnetic separators and the bins and necessary accessory apparatus. In many cases the equipment and operation expense does not make the separation game worth the candle.
Electrostatic Separation.—Electrostatic separation is due to the work of Blake and Morscher. The improved Huff machine has an all metal frame and static electricity is provided by a 4 hp. motor-generator set, the generator being rated at 300 volts and 5 amp. The transformer used delivers an alternating current to a revolving rectifier or interrupter mounted on the shaft of the generator. By this rectifier either all the crests of the alternating current wave, which are of course, of like sign or all the troughs could be used by the separator if the rotation of the apparatus is reversed. The voltage at the separator is between 20,000 and 30,000. The actual separating unit of the device is a statically charged round metal bar of small diameter. This can be used stationary or revolving if a wide spread of repelled material is desired. In either case the material to be separated is delivered at its top and slides down over the curved face. Opaque substances with a metallic luster are repelled and earthy materials are attracted. Some of the more common minerals which are repelled (conductors) are native metals, pyrite, chalcopyrite, galena and other sulphides. Even hornblende since it has a lustrous surface and some metallic content is repelled though feebly. The non-conductors are the earthy materials and gangues and such substances as pure zinc sulphide or resin jack which has no metallic luster. Resin jack has been made a conductor by treating it with a weak solution of copper sulphate.